This invention relates to the establishment of laminar boundary layer flow on an aerofoil body. In particular, the invention is concerned with maintaining a laminar flow along the attachment line of an aerofoil body with a leading edge inclined to the fluid flow direction, such as a swept back wing or fin. The invention also extends to other aerodynamic or hydrodynamic aerofoil bodies such fins, and bows and keels of watercraft, swept forward or backward.
It is desirable to decrease skin-friction drag on aerodynamic surfaces in order to reduce fuel costs and thus pollution caused by fuel burn. Laminar boundary layers create smaller frictional drag forces on an aerofoil body than turbulent flows. There are therefore significant advantages to be gained by maintaining or re-establishing laminar flow over as much of the surface of the body as possible.
Various proposals exist for maintaining laminar flow over an aerofoil, such as surface suction or surface cooling. However, the success of these forms of active laminar flow control are contingent upon the boundary layer on the attachment line being in a laminar state. The natural boundary layer on the attachment line of a swept wing will be either laminar or turbulent, depending on the value of the Reynolds number of the attachment boundary layer. If this parameter is sufficiently small the flow will be laminar, but above a first, or lower, critical value turbulent flow from the root region will propagate outboard and contaminate the entire attachment line causing the boundary layer there to be turbulent. This is because there tends to be a spanwise component of air flow travelling along a swept back wing from the root to the tip. When the Reynolds number is above some higher second critical value, the boundary layer on the attachment line is unstable and will develop into a turbulent state.
The flow over the swept wings of most transport aircraft tends to be in the intermediate range between the first and second critical values, such that the flow tends to be turbulent unless some action is taken to prevent contamination from the turbulent flow that inevitably forms at the wing root.
There have been various proposals to prevent or mitigate the effects of leading edge boundary layer contamination. In one proposal, laminar flow along the leading edge is re-established by locally reducing the leading edge radius with a bead, thus reducing the Reynolds number on the attachment line. This method makes the boundary layer revert to a laminar state, but the reduced leading edge radius inevitably affected the aerofoil performance in various ways.
U.S. Pat. No. 3,288,399 describes an arrangement in which a shaped “bump” having a bluff front end and an inclined rearward surface is fitted to the leading edge. The bluff front creates a stagnation region whereby a laminar boundary layer is established on the rearward surface. The “bump” has been used successfully in both wind tunnel and flight experiments but a suitable shape can only be obtained by wind tunnel experiments. However, it does appear that there is a Reynolds number limit beyond which the bump fails to prevent contamination.
Whilst laminar flow may be obtained by providing suction openings along the leading edge, this requires a very large amount of suction and is so complicated and expensive that the benefits are not seen to outweigh the disadvantages.
There is therefore a need for an arrangement capable of re-laminarising the boundary layer flow along the attachment line which overcomes or mitigates at least some of the disadvantages of existing arrangements. This invention aims to establish laminar flow by simple means which initiates a fresh laminar boundary layer on the attachment line and disposes of the turbulent boundary layer flow, without providing a gross disturbance to the flow over the body.